1. Field of the invention
The present invention relates to a knocking control system for an internal combustion engine, more specifically to a knocking control in response to an octane number of a fuel for the engine.
2. Description of the Prior Art
There have been proposed various controls for controlling an ignition timing, air-fuel ratio, supercharging pressure of intake air, compression ratio, and the like so as to suppress the knocking which is apt to cause engine troubles.
In U.S. Pat. No. 4,387,571, there is disclosed a knock control in which the supercharging pressure for intake air is changed in accordance with the frequency of the knocking occurrence. Japanese patent public disclosure No. 58-167881 laid open to the public on Oct. 4, 1983 discloses a knock control in which when the retard amount of the ignition timing is more than a predetermined value, the supercharging pressure is reduced. Further, Japanese patent public disclosure No. 61-16239 laid open to the public on Jan. 24, 1986 discloses a knock control in which the supercharging pressure is determined based on the ignition timing compensated in accordance with the knocking occurrence.
There has also been proposed an engine which is available for different octane number of fuels. The octane number of the fuel affects an occurrence of the knocking so that the knocking control is necessary to be changed in accordance with the octane number of the fuel.
In the above type of engine to which different octane number of fuels are supplied as disclosed in Japanese patent public disclosure No. 60-75730, which is laid open to the public on Apr. 30, 1985, the knocking is detected by detecting an engine vibration and the like so that the octane number of the fuel supplied to the engine is judged from the occurrence of the knocking and the ignition timing is controlled in accordance with the octane number wherein the ignition timing is retarded as the number of the knocking occurrence is increased. In this control, at first the ignition timing is set for a high-octane fuel. And, when a knocking occurs, the ignition timing is retarded. Even when the ignition timing is retarded by a certain time and the knocking is not eliminated, it is judged that the fuel is not a high-octane fuel not a regular fuel. In addition, in the knocking control disclosed in the above Japanese application, a supercharging pressure is reduced when the fuel is judged as a regular gasoline.
Therefore, once the octane number is judged and the ignition timing is set, the timing is usually maintained until the engine is stopped because it is unlikely to change the octane number of the fuel during a running of a vehicle. Therefore, where the judgment of the octane number is not correct, the ignition timing could not be set properly.
It should be noted thhat a knocking sensor is apt to pick up a noise other than a noise signal denoting a knocking and that such mis-judgment of the octane number is likely to happen. Under these circumstances, in order to judge the octane number of the fuel correctly using such knocking sensor, it is necessary to set the ignition timing with a substantial delay time enough to distinguish one another. However, as the delay time in the ignition timing is increased, it takes more time to reach the set ignition timing through the knocking control. When the ignition timing attains to the set value and the knocking is not eliminated, the supercharging pressure is reduced. Nevertheless, where the knocking is not eliminated, the control is necessary to be continued repeatedly. This means that the knocking control is less responsive. As a result, it takes a long time to obtain a proper ignition timing so that the knocking occurs continuously to cause troubles such as an increase of an exhaust gas temperature.
In the knocking control aforementioned, although the ignition timing and supercharging pressure are utilized for judging the octane number of the fuel and for eliminating the knocking, the knocking control results in the same even when the other factors such as air fuel ratio are employed for judging the octane number of the fuel and for suppressing the knocking.
As aforementioned the above knocking control is disadvantageous in that the judgment of the fuel octane number takes long to deteriorate an engine output and fuel consumption efficiency.